1. Field of the Invention
The present invention relates to the field of dry platelet preparations. More specifically, the present invention relates to dry platelet preparations and their use in tissue regeneration and as a hemostat for clotting of wounds.
2. Description of Related Art
Platelets are formed in the bone marrow as fragments of megakaryocytes. They are irregularly-shaped, colorless bodies that are present in blood at a concentration of 150,000-450,000 per microliter (ul). Platelets play a crucial role in hemostasis, and they are the first line of defense against blood escaping from injured blood vessels. When bleeding from a wound suddenly occurs, the platelets gather at the wound and attempt to block the blood flow by forming a clot. There are two general mechanisms to clot formation. In one mechanism, a clot begins to form when the blood is exposed to air. The platelets sense the presence of air and react with fibrinogen to begin forming fibrin. The resulting fibrin forms a web-like mesh that traps blood cells within it. In the other general mechanism, damaged blood vessels release a chemical signal that increases the stickiness of platelets in the area of the injury. The sticky platelets adhere to the damaged area and gradually form a platelet plug. At the same time, the platelets release a series of chemical signals that prompt other factors in the blood to reinforce the platelet plug. Between the platelet and its reinforcements, a sturdy clot is created that acts as a patch while the damaged area heals.
Platelets, in the form of platelet gels, have been used extensively to accelerate wound healing and, in conjunction with autologous fibrin glue, autologous platelet gel has been shown to improve perioperative hemostasis and reduce blood transfusion needs in surgery to replace the ascending aorta (Christenson and Kalangos, 2004). Costasis Surgical Hemostat (Costatis®). A combination of bovine thrombin, bovine collagen, and plasma as the source of fibrinogen and platelets has been shown to work well in the in vivo bleeding rabbit kidney and spleen model (Prior et al., 1999). Nevertheless, other studies have shown that platelet gel, when used alone, is not an effective hemostasis agent (Wajon et al., 2001). Despite of the contradicting findings regarding platelets and their role as hemostasis agents, there is little doubt about the pro-coagulant nature of platelet microparticles; these essential components, often overlooked, are increasingly being recognized as active participants in the in vitro and in vivo clotting process (Nieuwland et al., 1997). When platelets are stimulated with a combination of physiological agonists, such as thrombin and collagen, they release large quantities of microparticles (Sims et al., 1988; Tans et al., 1991). The activated platelets and microparticles express an aminophospholipid, which provides a procoagulant surface to support the formation of activated clotting enzymes in the intrinsic, extrinsic, and common pathways (Rosing et al., 1985).
Compared with activated platelets, microparticles contain a higher density of high-affinity binding sites for activated factor IX (IXa) (Hoffman et al., 1992) and factor Va (Sims et al., 1988). They have a continuous expression of high-affinity binding sites for factor VIII (Gilbert et al., 1991) and support both factor Xa activity (Gilbert et al., 1991; Holme et al., 1995) and prothrombinase activity (Sims et al., 1989).
Aside from the fact that platelet microparticles are important components in the hemostatic response, platelets, in the form of platelet gels, have been used in surgical wound healing applications as well as to treat difficult to heal wounds (Mazzucco et al., 2004). Moreover, the use of platelets in the form of platelet rich plasma has expanded into novel applications, such as bio-tissue engineering or autologous and allogenic tissue grafts, as well as osseous bone integration and soft tissue regeneration (Oikarinen et al., 2003). This is because platelets contain a number of important growth factors within their alpha granules that contribute to the process of hemostasis and wound healing. Studies have found that growth factors, such as platelet derived wound healing factors (PDWHF), platelet-derived growth factor (PDGF), transforming growth factor (TGF), and insulin growth factors (IGF), among others, are important in different stages of the wound-healing cascade and greatly influence mitogenic and cellular differentiation activities (Pierce et al., 1989; Steed, 1997).
These findings have lead to the development of strategies for growth factor replacement. For example, Regranex®, a recombinant human PDGF in a carrier gel, is used to treat diabetic wounds, while others, such as TGF, are currently being tested for FDA approval. Nevertheless, a single growth factor applied into a wound is not as effective as multiple growth factors. This is not surprising since wound healing is a complex integration of cascades that requires multiple growth factors for different stimulatory and inhibitory functions at different phases within the process.
Even though numerous advances in blood products and wound healing have taken place over the last several years, there is still a need for improved compositions for treating wounds, such as by hemostasis or clotting of wounds. Likewise, there is a need for methods of treating wounds to stop blood loss that are rapid, effective, and suitable for use in multiple settings.